1. Field of the Invention
This invention relates to the removal of the liquid and vapor content (inventory) of a storage tank and, more particularly, to a liquid storage tank having a floating roof.
2. Description of the Prior Art
Floating roof storage tanks are designed so that the roof of the tank floats on top of the liquid inventory stored in the interior of the tank, and floats downwardly toward the bottom of the tank as that liquid is withdrawn from the tank for other disposition.
The floating roof has at least one dynamic seal between it and the tank wall or walls to prevent vapor from the liquid inventory from escaping to the ambient atmosphere that is present over the roof and outside the tank's interior.
Such a roof carries with it a plurality of legs which stop the roof and hold it stationery a finite distance above the tank bottom so that once the liquid is essentially completely removed from the tank, workmen can enter, through man ways, the tank's interior below the stationery roof. The workmen can then carry out cleaning and/or maintenance projects on the interior sides of the tank's walls, bottom, and/or roof. Thus, for this purpose, the roof can be stopped and held as much as seven feet apart from and above the bottom.
When the roof is first brought to rest on its support legs, a substantial volume of liquid and some related vapor remains in the tank, both of which must be removed before workmen can gain access to the interior of the tank while the roof is held stationery.
Simple removal of this remaining liquid tends, because of the seal between the roof and the tank walls, to create a vacuum within the tank which can damage the tank and/or roof itself if the potential to form a vacuum is not addressed and controlled.
Heretofore, vents carried by the roof were opened while liquid was removed from the interior of the tank under the stationery roof in order to allow ambient air to enter that interior, equalize the pressure therein, and otherwise prevent the formation of a vacuum therein. When this technique is used, vapor, particularly vapor from inventory liquid, escapes to the ambient atmosphere during the entire time period it takes to essentially fully de-inventory the tank. Such a time period can extend to several days because such tanks can be so large in interior volume and the roof becomes stationery a number of feet above the tank bottom. Thus, a substantial volume of vapor can enter the ambient atmosphere during this final emptying or de-inventorying step. This can be unacceptable under the then existing emissions standards for the location and nature of the stored liquid inventory, and particularly when such liquid is hydro carbonaceous in nature.
It is, therefore, desirable to be able to essentially completely de-inventory a floating roof tank with minimal emission of interior tank vapor to the ambient atmosphere around the tank, and without damaging the tank or roof in the process.